Avian lungs have a unique unidirectional flow pattern, airflow being directed by 'aerodynamic valves'. The functioning of these valves is heavily dependent upon inertial forces in the gas. Such forces are also important in mammalian lung under some circumstances (e.g., high frequency ventilation and panting). The avian respiratory system provides advantages for study because it has large gas reservoirs, separate from gas exchange regions. This enables measurement of pressures and gas concentrations at various sites in the system, and tracing of gas flow routes which would be difficult or impossible to do in mammalian lung. While the existence of aerodynamic valves in bird lung has been recognized for half a century and flow routes through the airways have been well described, there is little understanding of the actual fluid dynamic mechanisms by which the aerodynamic valves in avian lungs direct flow. We will (1) characterize the physical parameters (gas velocity, gas density and viscosity, steadiness of flow) which affect the valves by injecting and sampling tracer gas at various sample sites in the goose respiratory system to determine flow routes. (2) use anatomical and gas flow data to develop physical and computational models which describe essential features of the valves, and (3) investigate the effect of variation in physiological conditions on the performance of valves.